In some situations, it is desirable to vary the intensity of light emitted from lighting, such as in stage production areas, auditoriums, meeting rooms and domestic scenarios. To vary the intensity of the light, a power dimmer can be used. A power dimmer is a mains powered device which, under command from an external control input, will control the intensity of a tungsten (filament) lamp. This control is commonly achieved using a technique called phase control, where the mains alternating current (AC) waveform is applied to the load for a time proportional to the required intensity of the lamp. For a bright lamp, the mains voltage is applied for the full ½ sine wave of each successive half-cycle of the mains AC cycle. For a dim lamp, a lesser portion of the half cycle is used. The tungsten lamps commonly used today are largely resistive and the dimmer is designed to drive this without issue.
LED lamp loads are commonly more reactive than tungsten lamp loads, with the LED device capacitance being the contributing parameter. There are trace inductances and resistance in LED lamps but the capacitance is generally a significantly greater contributor to the device reactance.
A regular resistive lighting load is predominantly resistive, so the current and voltage waveforms are generally in phase at all times. However, when dimming an LED light, the current and voltage become out of phase due to the capacitance of the LED. When the AC voltage waveform is applied to the LED, the internal device capacitance charges up to the applied voltage and remains there. When the voltage drops and passes through zero (at the zero crossing point of the AC mains voltage), the charge of the capacitance begins to flow back into the dimmer output. This causes the current waveform to reverse and causes the Triac (which is an AC switching device for the dimmer output stage) to retrigger and continue to conduct into the next ½ mains cycle. This causes the LED to receive full line voltage so the LED flickers to full brightness. This is known as a “flicker to full”. This problem becomes more prevalent with more LED loads being connected to a single drive circuit, as the bulk capacitance can increase to the point where the Triac will not be able to turn off.
The second issue with an LED is that it represents a very small load (current/wattage). A typical small-filament lamp will draw around 60 W, for example, which is fine for most dimmers. However with a small load, as with an LED, some dimmer circuits will have trouble making the Triac switch on and remain switched on. The reason for this is that most dimmers are fired by optoelectric devices and the circuit configuration is such that the Triac depends on the lamp loading to properly switch on the Triac. If the load is too small, the Triac will not latch and the output of the dimmer will fall back to zero volts after being activated. This is known as a “flicker to off”.
The problems described above are that too small a load is fed from an optoelectronic coupler-fired dimmer, which causes the Triac to not latch on, and too large an LED load (due to too much bulk capacitance), which causes the Triac to re-fire sporadically. The combined effect of these two conditions is to cause an LED dimmer circuit to misfire in various situations or to cause the LED to not be able to turn off.
It is desired to address or ameliorate one or more shortcomings or disadvantages of prior dimmer circuits for LED loads, or to at least provide a useful alternative thereto.
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Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.